GRAMMER

2 answers

1

None of them are sentences.

That said, to be "on call" is an American (English? I don't know where else it is used.) idiom used to describe a state of being available for work if necessary or in case of emergency. As in, "My friend the paramedic is always nervous when he's on call." By extension, it may refer to any period of time in which the subject is waiting for imminent news.

"Online" or, I think, "on-line" is another idiom meaning a state of being connected to the internet, particularly with a social chat application. This is not used as often now that telephone and internet do not conflict for most users.

However, if you intend these phrases to be synonyms meaning actually engaged in a telephone conversation, then none of them are correct; they are all missing a participle. "John is on a call." "John is on a/the phone." "John is on the line."

That's right, none of them are grammatical sentences, but each might be used (and understood) as an abbreviated message – such a casual approach would be acceptable only to people you are very familiar with.

"On call" has exactly the same meaning in British English.

"Online" has no hyphen.

If you are asking which correctly applies to being in a phone conversation with somebody, it's "I'm on the phone".